1. Field of the Invention
The present invention relates to a video data recording/reproducing apparatus, and more particularly to a video data recording/reproducing apparatus for subjecting information signals to the low bit rate coding process and then recording and reproducing the same with respect to a predetermined recording medium.
2. Description of the Related Art
In the prior art, as a method of magnetically recording and reproducing an audio signal or video signal, an analog signal recording system such as a bias recording system or FM (frequency modulation) system has been used. As an apparatus for magnetically recording analog signals, various types of VTRs (video tape recorders) meeting the requirements of standards of VHS, .beta. and 8-mm are provided. Recently, in order to attain the high quality sound and high quality image, apparatuses of digital recording system causing less signal deterioration at the time of recording or reproducing operation are commercialized. As an apparatus for magnetically recording digital signals, a DAT (digital audio tape) for recording audio signals and broadcasting D-1, D-2 VTRs for recording video signals are provided.
FIG. 25 is a block diagram showing a conventional video data recording/reproducing apparatus constructed by the above-described digital VTR. Unlike the broadcasting VTR such as the D-1, D-2 VTR, the apparatus shown in FIG. 25 is formed for use in the industrial-consumer field (home use) and the amount of information is reduced by the low bit rate coding process to attain the long-time recording operation.
An analog input video signal is converted into digital data in an analog/digital (A/D) converter 1 and then supplied to a format conversion circuit 2. The format conversion circuit 2 converts the input interlace signal into a non-interlace signal, converts the signal into a preset processing unit (for example, for each block structure of 8 pixels.times.8 scanning lines) and outputs the same. Data from the format conversion circuit 2 is input to a compression circuit 3 for each processing unit and subjected to the low bit rate coding process.
The amount of data output from the compression circuit 3 is a fraction to several thousandths of the input data amount. A parity code is added to data from the compression circuit 3 in an error correction code adding circuit 4. The parity code is set to correspond to a random error and burst error by taking the error generation state of the magnetic recording medium into consideration. The parity added data is input to a modulation circuit 5, converted into a code which is suitable for recording and then supplied to an adder 6.
Like the video recording system, an audio signal recording circuit 7 subjects an input audio signal to the A/D conversion and low bit rate coding process, adds an error correction parity to data, records, modulates and outputs the data to the adder 6. The video and audio data items are multiplexed on the time base by means of the adder 6 and then supplied to a recording amplifier 8. The recording amplifier 8 supplies data to a video head 9 which in turn magnetically records the data on a magnetic tape 10.
At the time of reproducing operation, data read out from the magnetic tape 10 by a video head 11 is distributed to the video circuit system and audio circuit system via a reproducing amplifier 12. A reproducing equalization/synchronization circuit 14 of the video circuit system equalizes the reproduced output and restores the output to digital data of synchronizing signal unit. The digital data is demodulated in a demodulation/TBC (time base correction) circuit 15 and output after the time base thereof is corrected.
The demodulated output is supplied to an error correction circuit 16 which in turn corrects an error included in the reproduced demodulated output and then outputs the output to an expansion circuit 17. The expansion circuit 17 demodulates (expands) an input low bit rate code to restore a signal of original data rate. The thus decoded signal is restored into a format suitable for output form such as display output form in a format conversion circuit 18, converted into an analog signal by a digital/analog (D/A) converter and output.
A reproduced signal from the reproducing amplifier 12 is also input to an audio signal reproducing circuit 13 of the audio circuit system. The audio signal reproducing circuit 13 has substantially the same construction as that of the video circuit system and functions to decode the reproduced signal, expand the signal so as to restore a signal of original data rate, and then convert the signal into an analog signal and output the same.
In FIG. 25, data is transferred between analog input and output terminals via the A/D converter 1 and D/A converter 19. In this case, the signal is deteriorated by the A/D conversion and D/A conversion. Therefore, in the Japanese Electronic Industries Association Standards CP-340, the digital interface standard "digital audio interface" in the audio system is defined. The above standard is adopted in the DAT, CD (compact disk) and the like. Therefore, a digital signal can be transferred without passing through A/D and D/A converters and data can be edited without deteriorating the signal when the data is edited while dubbing the tape.
FIG. 26 is a diagram for illustrating a format of the above standard.
In the above standard, the format sampling frequency, input/output connectors and the like are specified. As shown in FIG. 26, one word is constructed by 32 bits, a maximum number of 24 bits including 4-auxiliary bits are used for audio data and 20 bits are normally used therefor, and the bits are transferred starting from the LSB (least significant bit) side. The other portion is constructed by a 4-bit synchronizing signal and respective bits of V, U, C and P. The V bit is a validity flag and indicates the presence or absence of an error of data. The U bit is a user bit and is used to transfer sub-code information inherent to respective devices. The C bit is a channel status bit and defines the quality of data in transmission. The P bit is a parity bit and is inserted to detect an error of data in transmission and always hold the synchronizing signal on the same polarity side.
Signals of L and R channels are transmitted while the signals on the L and R channels are being alternately multiplexed in a time-divided manner. The two-channel signals constitute one frame by use of three types of synchronizing patterns. Data of 192 frames constitutes one block.
The sampling frequency, quantization number, effective sampling number, multiplexing method for Y, Cr and Cb signals and the like are defined with respect to the video signal according to CCIR (Comite Consultatif International des Radiocommunication), Rec. 656 (SMPTE, RP-125) (described in Television Institution Papers Vol. 40 No. 6, 1986 p442 to p448 "CCIR Digital Television 4:2:2 Interface").
Thus, on the transmission side, to-be-transferred data is reconstructed in a preset format and then transmitted. As an error correction code in the transmission path, only a code of simple configuration can be used. On the other hand, on the reception side of the magnetic recording apparatus, an error correction code is constructed to correct the recording error. That is, it is necessary to form an error correction system on the reception side irrespective of the transmission side. Thus, an error correction code different from that on the transmission side is constructed on the reception side and the data rate is different from that on the transmission side, thus making the apparatus extremely irrational.
For the low bit rate coding process for television conferences and television telephones using a 64-Kbps circuit of ISDN (Integrated Services Digital Network), a CCIR H.261 recommendation is reported. FIG. 27 is a view for illustrating the conventional video data recording/reproducing apparatus used in the television (TV) conference. The apparatus of FIG. 27 uses a DT640 coding/decoding unit (made by TOSHIBA).
A camera 21 for photographing participants and a monitor TV 22 for displaying the state of a TV conference hall B are installed in a TV conference hall A. An output signal of the camera 21 is input to a coding/decoding unit 23 (VIDEO CODEC DT64), subjected to the low bit rate coding process and then output from a transmission/reception circuit 24 (MK-6000). As a transmission path, a high-speed digital circuit 25 or communication satellite 26 may be used.
On the side of the conference hall B, data received in a transmission/reception circuit 27 (MK-6000) is decoded by a coding/decoding unit 28 and then displayed by a large projector 29, monitor TV 30 and the like. Thus, the state of the conference hall A can be observed in the conference hall B. Further, the state of the conference hall B is photographed by a camera 31 and displayed on the monitor TV 22 in the conference hall A via a path in a direction opposite to that described above. As a result, a conference can be held between the conference halls A and B which are located in the remote areas. Further, the audio signal can be transferred in the same manner as the video signal.
When a photographed video image is recorded, a signal decoded by the coding/decoding unit 23 or 28 is restored to the base band signal and then supplied to a VTR. In this case, only a video image photographed by one of the cameras can be supplied to a corresponding one of the VTRs. Therefore, at least two VTRs are necessary in order to record the states of the conference halls A and B. Further, as the number of cameras is increased, the number of VTRs for recording video images from the cameras must be increased, and it is necessary to operate the camera and VTR in synchronism with each other. Since a large number of tapes are used to record the proceedings of the conference, the running cost will be increased.
As described before, in a digital VTR for recording and reproducing digital information, an error correction code is added thereto to correct an error caused in the recording/reproducing operation and then the digital information is recorded. Generally, a product code of Reed-Solomon (which is hereinafter referred to as an RS code) is used as the error correction code.
FIG. 28 is a diagram showing the construction of the RS code.
As shown in FIG. 28, in the above correction system, a first correction series (P series) 37 constructed by a data series 35 and checking code series 36, a data series 38 having data items in the P series as constituents and defined by different series and a second correction series (Q series) 40 constructed by a checking code series 39 are created in the same data space (product code block) and recorded. At the time of reproducing operation, the P series is first used to detect and correct errors and then the Q series is used to correct errors of data which cannot be corrected by use of the P series. In this case, a high correction ability can be attained at the time of correction for the Q series by use of error detection information of the P series.
Recently, the high-definition broadcasting is now on the point of starting. In the high-definition broadcasting, an amount of information to be transmitted is extremely larger than that in the present NTSC (National Television System Cominitee) broadcasting. Therefore, in order to record information in both of the systems, it is considered to variably change the inclination angle of the tracks so as to change the length of the track according to the system. FIGS. 29A to 29D are diagrams used for illustrating recording tracks of this type of conventional video data recording/reproducing apparatus.
FIGS. 29A to 29D respectively show the recording tracks for NTSC signal, EDTV (Extended definition Television) signal and HD (High Definition Television) signal and the recording capacity thereof. If the track length of one of recording tracks 41 of the HD signal shown in FIG. 29C, the track lengths of recording tracks 42 and 43 of the EDTV and NTSC signals are L/2 and L/3. Thus, the recording capacity for each track is changed by changing the inclination angle of the track to change the track length.
However, since the recording capacity of the recording tracks 42 and 43 for NTSC and EDTV signals is smaller than that of the recording track 41, the Q-series signal cannot be completely recorded on the tracks 42 and 43 (refer to FIG. 28) when the RS code is constructed according to the amount of information of the HD signal. Then, when the NTSC and EDTV signals are reproduced, the error correction is effected only in the P series and the correction ability is significantly degraded. Further, when RS signals are constructed for respective formats which are different for respective systems, the circuit scale becomes extremely large.
In the D-2 digital VTR system and the like, a recording format shown in FIG. 30 is used.
As shown in FIG. 30, in the D-2 format, data is constructed by a 62-byte track preamble T, 6-sink block (=1140 bytes) audio sectors A0 to A3, 6-byte post-amble P, 156-byte editing gap EG, 28-byte editing gap preamble E and 204-sink block (=38760 bytes) video sector v.
FIG. 31 is a block diagram showing the recording side of the conventional video data recording/reproducing apparatus using the above-described D-2 format.
An input audio signal input via an analog/digital interface 45 is divided into blocks by an audio datablock forming circuit 46 and then supplied to an external code forming circuit 47. The external code forming circuit 47 adds an external code (Q code) to the input data and then supplies the same to an audio data shuffling circuit 48 which in turn shuffles the audio data and outputs the same to a data multiplexing circuit 53. An input video signal is supplied to a channel distribution circuit 50 via an analog/digital interface 49 and divided into blocks for each channel. An external code (Q code) is added to data divided into blocks in an external code forming circuit 51 and then the data is shuffled in an in-sector shuffling circuit 52 and output to a data multiplexing circuit 53. The data multiplexing circuit 53 is supplied with a synchronizing signal and ID signal from a synchronizing ID circuit 54, and the audio and video signals which are separately processed are subjected to the time-division multiplexing process in the data multiplexing circuit 53. Further, an internal code (P code) is added to the data in an internal code forming circuit 55 and then the data is subjected to the channel-coding process in a channel coding circuit 56 and output to a recording amplifier 57. The recording amplifier 57 amplifies the input data and records the data on the helical track for each channel.
As shown in FIG. 30, editing gaps E are provided between the audio and video signals and between the audio signals. The editing gap E permits the audio and video sectors to be independently edited and the preamble P and post-amble P lying before and after the editing gap E are used to make the pull-in and reproduction of a bit synchronization clock easy. In a digital VTR of D-1 format, the audio and video signals are separately processed and external codes are added thereto and they are commonly processed for the internal code. The D-1 and D-2 formats are provided for business use and no audio multi-track format is provided.
FIG. 32 is a diagram for illustrating the format of a multi-track system used in an 8-mm VTR. As shown in FIG. 32, in the PCM (pulse code modulation) audio multi-track system, first to sixth audio tracks are formed in one track by use of the format of PCM audio part. In this case, the video part is FM-recorded and has no relation with respect to the format of the audio part.
Thus, the video and audio signals which are separately processed are separately recorded with the editing gaps on a tape and are subjected to different signal processings. Since the audio and video signals are subjected to different signal processings, the circuit scale must be made large. Further, when a plurality of only audio signals are recorded on one track in the format of audio part, the formats of the audio and video parts are different from each other and different formats will be present on the tape. Therefore, those of the ID and sub-code portions to be referred at the searching time or the like which can be reproduced is limited.
In a recording/reproducing device having a large recording capacity, the searching function is an indispensable additional function. The searching operation is required to be effected with high precision and high operation speed. For example, in a VTR, in order to enhance the searching precision, the searching operation is effected for each frame or field unit instead of each unit of minute or second. Further, in order to enhance the operation speed, that is, move the head to a preset tape position in the shortest possible time, means for correctly detecting an index signal must be provided in addition to means for feeding the tape at a high speed.
FIGS. 33A and 33B are diagrams showing track patterns used in a DAT. FIG. 33A shows the locus of a head at the time of high searching operation and FIG. 33B shows the output waveform of an RF signal at the time of high searching operation.
In a DAT having no fixed head, a sub-code track area is provided in the inclined track and a sub-code in the sub-code area is read out by use of a rotating head at the time of high-speed searching operation. As shown in FIG. 33A, the head traces several tracks by one scanning operation. As shown by signs of + and - in FIG. 33A, the azimuth angle varies for each track. When the head azimuth is +, the reproduced envelope shown in FIG. 33B is obtained. As shown in FIG. 33B, the reproduced envelope takes a form corresponding to a series of beads of a Japanese abacus and the period thereof is determined by the number of tracks scanned by one head scanning operation.
In order to derive a sub-code from the reproduced signal, it is only necessary to set the period of a portion of the reproduced envelope which is kept higher than a preset level to be equal to or longer than time required for the reproduction of the sub-code. As the searching speed becomes higher, the number of tracks to be scanned by one head scanning operation increases and the period of the reproduced envelope becomes shorter. Therefore, the searching speed of high-speed searching operation up to approx. 400 times the normal speed is considered to be the upper limit as is disclosed in "RADIO TECHNOLOGY" (APR.1987) when taking the pull-in of PLL (phase locked loop) for synchronization into consideration.
In order to reproduce the index signal at the time of high-speed searching operation, it is necessary to set the rotation speed of the drum to a preset value so as to set the frequency band of the reproduced signal equal to that obtained in the normal reproducing operation. It is necessary to set the rotation speed at the time of high-speed searching operation in a wider range (1000 to 3000 rpm) in comparison with that of the rotation speed (2000 rpm) in the normal reproducing operation. The rotation speed of the drum is changed by detecting and setting a point at which the clock PLL is locked.
Thus, it is necessary to change the rotation speed of the drum in a wide range in order to effect the high-speed searching operation. Further, the searching operation cannot be started until the pull-in of the PLL is detected, and therefore, when taking the signal processing time for the index signal into consideration, the index signal cannot be reproduced if an envelope output which is higher than the preset level cannot be obtained for a relatively long period of time. As a result, it becomes impossible to effect the searching operation of ultra-high speed which is 1000 times higher than the normal operation speed, for example.
There is also provided a system in which a linear track is provided in another area on the tape and an index signal is recorded in this area. However, since the reproduced signal band varies in proportion to the tape feeding speed, it becomes necessary to set the signal processing system for operation in the high frequency band in order to effect the high-speed searching operation, and in this case, the SN ratio is lowered. Further, when the tape feeding speed is significantly changed, a sufficiently large output may not be obtained because of the characteristic of the tape head system.
In the digital recording system, it is only necessary to identify "1" and "0" at the time of reproducing operation. Therefore, a reproduced signal of higher SN ratio in comparison with a case of the analog recording system in which the SN ratio of the reproduced signal from the magnetic medium influences the SN ratio of the final output can be obtained by use of the error correction technique based on the coding theory.
In the digital recording system, an analog signal is converted into parallel digital data, subjected to various processings, converted into serial data and then recorded. In this case, synchronizing data is inserted in order to identify the boundary portion of each parallel data at the time of reproducing operation. The synchronizing data is constructed by a pattern of several bits which can be separated from the pattern of main signal data and is generally inserted and recorded at regular intervals of several parallel data items.
The synchronizing data is first detected at the time of reproducing operation and is used to sequentially divide the main signal data according to the detected timing. Since the synchronizing data is added for every several parallel data items, the main signal of the several parallel data items cannot be detected when the synchronizing data cannot be detected and the possibility of an error to occur in the main signal data is high. However, unlike the main signal data, an error correction code for correcting an error of the reproduced data cannot be added to the synchronizing data. Therefore, when the synchronizing data is detected, a method for comparing the pattern of the reproduced data with the synchronizing pattern and determining that the synchronizing pattern is detected if a difference therebetween is smaller than a preset number of bits is sometimes used. Further, the synchronizing data is inserted at regular intervals, a method of interrupting the detecting operation for a preset period of time after detection of the synchronizing pattern may be used to prevent the synchronizing pattern from being erroneously detected.
In the digital recording operation, the necessary recording band is wider than that in the analog recording operation. Therefore, in the analog recording VTR, a video signal of one field is recorded on one track, but in the digital recording VTR, it is necessary to record a video signal of one field separately on several tracks. For this reason, in the digital recording operation, consumption of the magnetic tape for each unit period of time is larger than that in the analog recording operation. However, the track width can be made smaller as the magnetic head and magnetic tape are improved, and it becomes possible to provide a necessary recording time.
However, when the recording track width is reduced, the tolerance for the tracking error which is the relative position of the magnetic head with respect to the recording track is reduced. In particular, reduction in the tolerance becomes significant in the recording/reproducing operation of different devices, that is, in the compatible reproducing operation, and in this case, it is difficult to attain a high SN ratio. Therefore, the tracking control of high precision is necessary in the high-density recording operation.
FIGS. 34 and 35 are diagrams for illustrating the tracking control operation. FIG. 34 shows a case of recording operation and FIG. 35 shows a case of reproducing operation. In the drawing, oblique lines indicate the azimuth recording operation. In a system for recording and reproducing data on the magnetic tape by use of a rotating head like the VTR, the traveling locus (recording track 92) of a rotating head 91 on a magnetic tape 90 at the time of recording operation is determined by the traveling speed of the magnetic tape 90 and the rotation speed of the rotating head 91 (FIG. 34). At the time of reproducing operation, as shown by broken lines in FIG. 35, the tracking control is effected so that the rotating head 91 may correctly trace the recording track 92. That is, the tracking control is effected to set up, in the reproducing operation, the same relation between the tape traveling and the head rotation as that set up in the recording operation.
In the prior art, many systems for effecting the tracking control method are proposed. For example, as a system practiced in the helical scanning VTR, the control system used in the D or VHS system is provided. In this system, exclusive tracks are provided in the longitudinal direction of the magnetic tape to record control signals and control and set the phase of the reproduction controlling signal to a preset value. However, since the exclusive track is used, the recording density is low, and since the fixed head is used, the traveling condition of the magnetic tape tends to vary. Further, the tracking error signal cannot be created in the main signal recording track width direction and it is not suitable for the precise positioning control in the high-density recording operation.
A so-called hill-climbing control system which is used together with the above control system to control and set the amplitude of the reproduced main signal to the maximum value may be sometimes used. However, this system can be used in the analog recording VTR in which preset frequency signals are recorded at regular intervals, but cannot be used in the digital recording operation in which the amplitude of the reproduced signal cannot be set constant.
On the other hand, in the 8-mm system which is a relatively high-density recording system among the analog recording system, a pilot system for recording the tracking pilot signal on the main signal on the frequency multiplexing basis, comparing the levels of pilot signals reproduced from the adjacent tracks with each other in the reproducing operation and effecting the position control to set the levels equal to each other. This system is suitable for the high-density recording since the tracking signal is recorded on the same track as the main signal so that the recording density will not be lowered and the tracking error signal can be created in the main signal recording track width direction.
In the 8-mm system, pilot signals are reproduced from adjacent tracks with different azimuth angles. Therefore, it is necessary to select low frequencies with relatively small azimuth-loss effect as the frequency of the pilot signal. For this reason, as shown in FIG. 36, the pilot signal frequency is set to be lower than that of the FM-modulated luminance signal and low-frequency band conversion color signal which are the main signal.
FIG. 37 is a block diagram showing a separation circuit for separating signals in the VTR of 8-mm system.
A reproduced signal from a magnetic head 61 is supplied to a low-pass filter (LPF) 63, band-pass filter (BPF) 64 and high-pass filter (HPF) 65 via a preamplifier 62. The LPF 63 deals with the bandwidth of the tracking pilot signal as the pass band thereof, the BPF 64 deals with the bandwidth of the low-frequency conversion color signal as the pass band thereof, and the HPF 65 deals with the bandwidth of the FM-modulated luminance signal as the pass band thereof. Therefore, the pilot signal, low-frequency conversion color signal and FM-modulated luminance signal respectively shown in FIGS. 36B to 36D are derived from output terminals 66 to 68.
FIG. 38 is a block diagram showing a digital VTR using the tracking pilot signal.
A video signal is input to an input terminal 71. An A/D converter 72 converts the input video signal into a digital signal and outputs the digital signal to an error correction code adding circuit 73 which in turn adds an error correction code to the main signal and outputs the same to a digital modulation circuit 74. In the digital recording operation, the digital conversion circuit 74 effects the data re-arranging process to reduce the amplitude of the D.C component to low frequency component of the frequency spectrum of the recording signal as shown in FIGS. 39A and 39B since the D.C. transmission cannot be effected in the electromagnetic conversion system. Synchronizing data is added to the output of the digital modulation circuit 74 in a synchronizing data adding circuit 75, a pilot signal from a pilot generation circuit 77 is further added to the output in a recording circuit 76 and then the output is magnetically recorded on a tape 78.
At the time of reproducing operation, the FM luminance signal of the reproduced signal is separated by the HPF 65 and supplied to a reproducing circuit 82. An output of the reproducing circuit 82 is supplied to a PLL 81 and time-base correction circuit 83 to correct the time base and then supplied to a synchronizing data detection circuit 84. The synchronizing data detection circuit 84 detects the synchronizing data and a digital demodulation circuit 85 effects the inverted process of the operation effected by the digital modulation circuit 74 in the recording operation so as to demodulate the video signal. An error correction circuit 86 uses a correction signal to correct the error of the demodulated data and a D/A converter 87 converts the digital signal into an analog signal and outputs the same to an output terminal 88. Further, the reproduced signal is also supplied to the LPF 63 which in turn separates the tracking pilot signal from the reproduced signal and outputs the same to a tracking control circuit (not shown).
As shown in FIG. 39B, the pilot signal is recorded on the video signal on the frequency-multiplexing basis. As described before, the digital modulation circuit 74 reduces the amplitude of the low-frequency band component by effecting the modulation process, but the level of the low-frequency band component is relatively high in comparison with a case of the analog recording operation. Therefore, when the pilot signal is separated by the LPF 63 in the reproducing operation, the low-frequency component of the video signal is removed and the SN ratio of the reproduced main signal is lowered.
Even in this case, as described before, the error correction code is added to the main signal data, and even if an error occurs in the reproduced signal as the result of reduction in the SN ratio, the error can be corrected to some extent. However, since no correction code is added to the synchronizing data, the synchronizing data may not be detected if the number of reproduction errors is increased as the result of reduction in the SN ratio caused by separation of the tracking pilot signal. Then, an error may occur in the main signal data of several parallel data items, the error correction ability thereof is exceeded and the reproduced signal is extremely deteriorated.
In FIGS. 34 and 35, the recording track 92 is formed in a linear configuration. However, in practice, the recording track 93 on the tape 90 snakes as shown in FIG. 40 by the influence of the lead configuration of the rotation cylinder and the like. When the recording and reproducing operations are effected in the same device, no particular problem will be caused by the snaking of the recording track 93 since the traveling loci of the rotating head in the recording and reproducing operations coincide with each other.
However, in the compatible reproducing operation in which the reproducing operation is effected in a device different from the recording device, the traveling loci of the rotating head in the recording and reproducing operations will not coincide with each other as shown by broken lines in FIG. 41 by the influence caused by a difference in the lead configuration and the like. Therefore, it is impossible to correctly trace the recording track 93 by simply setting the relation between the tape traveling and the head rotation in the recording operation equal to that in the reproducing operation. Such a tracing deviation gives more serious influence as the track width is made smaller, and a sufficient reproduced envelope may not be sometimes obtained.
In order to solve the above problem, in the prior art, a video data recording/reproducing apparatus in which the tracking can be correctly effected by use of a movable head is proposed. FIG. 42 is a block diagram showing a dynamic tracking following (DTF) circuit used in the above apparatus and FIG. 43 is a diagram showing a head portion.
As shown in FIG. 43, a piezoelectric element 97 is mounted on an upper cylinder so as to be freely rocked and a rotating head 98 is attached to the end portion of the piezoelectric element 97. The piezoelectric element 97 can be displaced in a direction perpendicular to the rotation direction of the cylinder 95 as shown by an arrow in FIG. 43, thereby making it possible to move the head 98 in the track width direction.
As shown in FIG. 42, an adder 101 superposes a pilot signal of preset frequency on the main signal to be recorded. An output of the adder 101 is amplified by an amplifier 102 and then supplied to the head 98 via a selection switch 103. The head 98 records the main signal having the pilot signal superposed thereon on a magnetic tape 104. As described before, the pilot signal is constructed by a low-frequency band component.
At the time of reproducing operation, a reproduced signal from the head 98 is supplied to a preamplifier 105 via the selection switch 103 and amplified. An output of the preamplifier 105 is supplied to a reproducing circuit (not shown) as a reproduced signal and at the same time the pilot signal is separated from the reproduced signal by a BPF 106. A detector circuit 107 detects the pilot signal to detect the level thereof. The detector circuit 107 supplies an output which may cause the maximum detected level to a drive circuit 108 and displaces the piezoelectric element 97. As a result, the head 98 moves in the track width direction to correctly trace the recording track 93.
Thus, in the DTF circuit, deviation of the rotating head is detected according to the detected level of the pilot signal and the piezoelectric element is driven based on the result of detection so as to cause the rotating head to follow the recording track.
However, in the DTF circuit in which the analog pilot is multiplexed, the track deviation of the rotating head can be prevented, but since a pilot signal component is mixed into the reproduced signal, the final error rate will be degraded. Further, when signal components of frequencies lower than the pilot frequency are removed from the reproduced signal by use of the HPF or signal components of frequencies nearly equal to the pilot frequency are removed from the reproduced signal by use of the BPF so as to prevent the pilot signal component from being mixed into the reproduced signal, the recording main signal of frequencies lower than or nearly equal to the pilot frequency will also be removed. Further, the phase of the reproduced signal of frequency near the cut-off frequency of the above filter will be varied.
In the prior art, when a digital signal is recorded or reproduced on a recording medium such as a magnetic tape, a longitudinal recording method for creating a magnetic pattern parallel to the recording medium is used. Assume now that a digital signal which varies in a step form is recorded on the magnetic tape. Also, in this case, magnetic inversion of the tape cannot take an ideal step form. Therefore, the waveform of the reproduced digital signal projects upwardly at the central portion and spreads out towards both end portions as shown in FIG. 44.
The reproduced waveform is called a solitary reproduced waveform, and it can be considered to be substantially symmetrical in the case of longitudinal recording and the waveform can be approximated by the Lorentz function expressed by the following equation (1). EQU f=a/(a.sup.2 +t.sup.2) (1)
where t indicates time, and a indicates a pulse width coefficient and is expressed by a=W50/2T. T is a bit interval and W50 is the half-width of a pulse.
In the digital recording operation, the condition that the data series can be identified without causing interference between codes can be obtained if the amplitude of the solitary reproduced waveform in the identification point of another code is "0" as shown in FIG. 45. For this reason, an equivalent circuit is used to eliminate the interference of adjacent pulses in the identification point. That is, the equivalent circuit is constructed to output an output waveform h(nT) satisfying the following equation (2) at a timing nT when assuming that the bit interval is T. EQU h(nT)=0, .vertline.n.vertline..gtoreq.1 EQU h(0)=1, n=0 (2)
As the above equivalent circuit, a transversal filter shown in FIG. 46 or the like is used.
An input signal from an input terminal 111 is supplied to a delay line group 112 constructed by delay lines each having a delay time of the bit interval T. The input signal and outputs of the respective delay lines are supplied to a gain controller group 113 constructed by tapped gain controllers. The input signal is sequentially delayed by time T by means of the delay lines and a plurality of copy signals are input to the respective gain controllers. The gain controllers are supplied with tap gains, add the tap gains to the respective copy signals, and then supply them to an analog adder 114. The analog adder 114 adds the input signals together in an analog manner and outputs the result of addition to an output terminal 115. As a result, an output created based on the tap gain is derived from the output terminal 115. Thus, the equalization operation is effected by delaying an input signal to generate copy signals of the same waveform as that of the input signal, adding the tap gain to the copy signals and then adding them together. Therefore, when the solitary reproduced signal which is an input signal is asymmetrical, it becomes difficult to effect the equalizing process.
As described above, the signal can be identified by equalizing the reproduced signal. As the signal detection system, various systems are provided according to the method of equalizing the reproduced signal in a case where a solitary pulse "00100" is used as a recording signal.
FIGS. 47 and 48 are circuit diagrams of recording and reproducing systems using a PR (1, -1) system as the signal detection system and FIG. 49 is a timing chart for illustrating the operations thereof (a) to (e) in FIG. 49 indicate the waveforms of signals on respective points (a) to (e) in FIGS. 47 and 48.
An input signal shown in (a) of FIG. 49 is supplied to a pre-coder 116. The pre-coder 116 is constructed by a 1-bit delay circuit 117 and an adder 118 and supplies an output corresponding to the exclusive-OR of the input signal and the output signal of the 1-bit delay circuit 117. That is, the pre-coder 116 effects the operation of mod2 with respect to the input signal to derive a recording signal shown in (b) of FIG. 49. An output of the pre-coder 116 is amplified by a recording amplifier 119 and recorded on a magnetic tape 121 by means of a ring head 120.
At the time of reproducing operation, a reproduced signal reproduced by a ring head 122 is amplified by a preamplifier 123 and supplied to an equalizing circuit 124. The equalizing circuit 124 effects the above-described waveform equalization for the reproduced signal to output an equalized signal shown in (c) of FIG. 49. A three-value comparator effects the three-value detection for the equalized signal and outputs an identification signal shown in (d) of FIG. 49 to a PLL 126 and identification circuit 127. The PLL 126 extracts a clock from the identification signal and supplies the same to the identification circuit 127 which in turn uses the clock to identify the identification signals "1" and "-1" as "1". Thus, a reproduced signal shown in (e) of FIG. 49 is derived from the identification circuit 127. Since the the reproduced signal is identified as "001-100" with respect to the recording signal "00100", this system is called PR(1, -1) system.
It is considered that the recording density is further enhanced. However, if the recording frequency is enhanced to attain the high-density recording, the reproduced amplitude is reduced, the interference between codes increases and the SN ratio is degraded by the waveform equalization. Thus, the longitudinal recording is not suitable for the high-density recording.
FIG. 50 is a diagram for illustrating the above problem and shows a magnetic pattern in the longitudinal recording in a case where the wavelength of the recording signal is .lambda..
A magnetic pattern 132 having a length of .lambda./2 and a thickness of .delta. is formed in a horizontal direction by a head 133 on the recording medium surface on a base 131 of a recording medium 130. The magnetic pattern 132 is formed with the same magnetic poles of adjacent segments set to face each other, and when the recording wavelength .lambda. is reduced, the self-demagnetization factor N [.varies..delta./(.lambda./2)] becomes large and the demagnetization force Hd (=N.times.M) in a direction opposite to that of the magnetization M also becomes large. That is, as the recording density becomes higher, the self-demagnetization becomes stronger, making it difficult to effect the recording operation. Further, since the bit interval T is reduced by the high-density recording, the pulse width coefficient a becomes larger, and as a result, the solitary reproduced waveform more widely spreads towards the end portions thereof, thereby further degrading the SN ratio by equalization.
The apparatus is further miniaturized and the diameter of the rotation cylinder is reduced. Therefore, the operation of the tape feeding system becomes unstable and air may be inserted between the rotation cylinder and the tape. Then, the tape floats to cause a large space loss. The space loss Lsp has a relation expressed by the following expression (3) if the space amount is s and the recording wavelength is .lambda.. EQU Lsp.varies.exp (-s/.lambda.) (3)
As indicated by the expression (3), the space loss rapidly increases when the recording wavelength becomes shorter. For the above reason, in the digital recording in the longitudinal recording operation, it is considered that a signal having the wavelength of approx. 0.5 .mu.m may be the limit for recording.
When the vertical recording method shown in FIG. 51 is used, the high-density recording can be effected.
In the vertical recording, as shown in FIG. 51, a magnetic pattern 139 having a length kept at .delta. is formed on the recording medium surface on a base 136 of a recording medium 135 by use of a main magnetic pole 137 and an auxiliary magnetic pole 138. In this case, the self-demagnetization factor is expressed by N [.varies.(.lambda./2)/.delta.] and the demagnetization becomes smaller as .lambda. becomes smaller, thus making the recording state stable. That is, a large reproduced output can be derived in the high-density recording. Further, since the inversion of the magnetization can be rapidly effected, an output signal is obtained in the form of a pulse waveform of narrow width and this is effective for the high-density recording.
In order to effect the vertical recording, it is necessary to use a head for creating the distribution of vertical magnetic field whose intensity is high and which rapidly varies. For example, as shown in FIG. 51, a head having the main magnetic pole 137 and the auxiliary magnetic pole 138 facing each other with a recording medium disposed therebetween may be used. However, it cannot be used in a VTR or the like using a rotation cylinder to effect the recording/reproducing operation.
Even a ring head used in the VTR has a strong vertical magnetic field component created at the gap edge, and if the recording medium has the vertical orientation property, substantially the vertical recording can be effected. This is called the quasi-vertical recording and Co-Cr, Ba-ferrite or the like is used as the recording medium. Particularly, when Ba-ferrite is used, the conventional coating technique can be used and the vertical orientation degree can be easily controlled, and therefore, it is highly expected to be used as the high-density recording medium.
Thus, in order to effect the vertical recording by use of the ring head, it is necessary to use a recording medium of high vertical orientation degree. However, as is disclosed in "PEAK SHIFT CHARACTERISTICS FOR BARIUM FERRITE FLEXIBLE DISK DRIVE" (1987 DIGEST OF THE `INTER MAG CONFERENCE` AB-04) (reference document 1), when the vertical orientation degree of the recording medium increases, the degree of asymmetry of the solitary waveform becomes larger. As described before, in the equalization and signal detection, the waveform equalization is effected to set "0" at the adjacent identification point on the assumption that the solitary reproduced waveform is substantially symmetrical, but it is extremely difficult to equalize the solitary reproduced waveform of large degree of asymmetry into an identifiable waveform. Further, the SN ratio is degraded with an increase in the amount of waveform equalization.
Thus, in the above-described conventional video data recording/reproducing apparatus, it is necessary to construct an error correction system irrespective of the transmission side on the receiving side. For this reason, an error correction code different from that on the transmission side is constructed on the reception side and the data rate becomes different from that on the transmission side, thus making the apparatus extremely irrational.
When a video image photographed in a TV conference or the like is recorded, a decoded signal is restored to the base band signal and then recorded, and therefore, recording devices of a number corresponding to the number of the cameras must be used and the running cost increases.
If the recording capacity of one track is variable, the product code of error correction will not be completed in a mode different from the mode in which the recording capacity is maximum and thus the error correction ability will be extremely degraded. Further, if the product code is constructed for each system, the circuit scale will be extremely increased.
When video and audio signals separately edited are separately recorded on the tape with an edition gap disposed therebetween, different signal processings are effected for the video and audio signals. When different signal processings are effected for the video and audio signals, the circuit scale must be made large. Further, when a plurality of audio signals are recorded in the format of audio section on one track, the formats of the video part and the audio part become different from each other so that portions of different formats will be present on the tape. Therefore, a reproducible part of the ID and sub-code portion referred to in the searching operation will be limited.
In order to effect the high-speed searching operation, the rotation speed of the drum must be varied in a wide range. Further, the searching operation cannot be started until the pull-in of PLL is determined, and the index signal cannot be reproduced unless an envelope output is kept higher than a preset level for a relatively long period of time when taking the signal processing time for the index signal into consideration. Therefore, it is impossible to effect the searching operation of ultra-high speed which is 1000 times the normal speed, for example.
In the conventional video data recording/reproducing apparatus, since the tracking pilot signal is frequency-multiplexed on the video signal, the SN ratio of the reproduced signal is lowered by separation of the tracking pilot signal so that the synchronizing data may not be reproduced and the main signal may not be decoded.
Further, in the DTF circuit in which an analog pilot signal is multiplexed, the problem of track deviation of the rotation head can be solved, but the pilot signal component is mixed into the reproduced signal so that the final error rate will be lowered. Further, when an HPF is used to remove components of frequencies lower than the pilot frequency from the reproduced signal or a band elimination filter (BEF) is used to remove the signal component of frequencies near the pilot frequency in order to prevent the pilot signal component from being mixed into the reproduced signal, the recording main signal of frequency band lower than or near the pilot frequency will also be removed. Further, the phase of the reproduced signal of the frequency near the cut-off frequencies of the above filters will vary.
Further, in the conventional video data recording/reproducing apparatus, if the vertical recording is effected by means of a ring-shaped head using a vertical orientation recording medium in order to effect the high-density digital recording, the solitary reproduced waveform becomes asymmetrical, thus making it extremely difficult to identify the reproduced waveform.